The present invention relates to a continuously variable traction roller transmission.
One of the previously proposed continuously variable traction roller transmissions is disclosed, for example, JP-A 63-130953 published on Jun. 3, 1988. This known continuously variable traction roller transmission includes a continuously variable transmission unit having an input disc, an output disc, and a pair of traction rollers which come in frictional contact with the two, and performs control of a gear ratio by altering a contact state between the two discs and the traction rollers. A predetermined frictional force should be applied between the two discs and the traction rollers so as to allow transmission of torque therebetween. For that purpose, a pressing force is applied between the two discs and the traction rollers, and in an axial direction of a rotation shaft of the two discs. This pressing force is provided by a loading cam or cam roller which produces thrust in response to input torque. It is to be noted that since the pressing force of the loading cam is scarcely produced when input torque is in the vicinity of zero, a preload of the disc springs is in operation as an initial pressing force. That is, the disc springs are compressed to have a predetermined amount of deformation, thereby obtaining an elastic force which is applied to the two discs and the traction rollers. Compression of the disc springs is made by a loading nut screwed to the rotation shaft. An amount of screwing of the loading nut is adjusted to set constant the preload provided by the disc springs. This adjustment is carried out by setting an axial clearance between the loading nut and a disc member which is arranged opposite to the side of the loading nut through the disc springs. That is, the loading nut is screwed until the clearance between the disc member and the loading nut comes to a predetermined value.
However, with such known continuously variable traction roller transmission, one problem is that the operation for adjusting and screwing the loading nut at the same time to obtain a predetermined clearance between the disc member and the loading nut is difficult to carry out and takes much time. Another problem is that a preload of the disc springs is difficult to set to a constant value due to variations susceptible to produce in adjusting operation.
The preload of the disc springs may be used in screwing the loading nut to come in contact with the disc member, viz., in a state where the disc springs have no margin of deformation. In that event, however, there arises the other problem that the pressing force exceeds a required level.
Referring to FIG. 2, a minimum pressing force required to transmit torque is indicated by a two-dotted chain line, where input torque is taken on the horizontal axis, and pressing force is taken on the vertical axis. If the pressing force is above the two-dotted chain line, torque can be transmitted, whereas, if it is below the two-dotted chain line, slippage can occur. A pressing force as obtained by the disc springs and the loading cam is indicated by a fully drawn line in FIG. 2. Specifically, when input torque is null, a load f.sub.0 as provided by the disc springs is in operation. As input torque increases gradually, the disc springs begin to be compressed with a force scarcely changed from f.sub.0. When the disc springs are compressed until there is any clearance between the loading nut and the disc member, a pressing force of the loading cam begins to operate due to the disc springs being incapable of further compression. Thus, a characteristic of pressing force as indicated by the fully drawn line in FIG. 2 is obtained. On the other hand, when no clearance is established between the loading nut and the disc member in screwing the loading nut to come in contact with the disc member from the beginning, viz., when the disc springs have no margin of deformation, a characteristic of pressing force as indicated by a broken line in FIG. 2 is obtained: When input torque begins to increase from zero, the pressing force of the loading cam becomes effective immediately due to the disc springs being incapable of further deformation at that moment. Of course, the pressing force as indicated by the broken line in FIG. 2 permits torque transmission, but it exceeds a required level. This results in inconveniences such as a decrease in durability of components, a decrease in transmission efficiency due to increased frictional resistance, etc.
Therefore, an object of the present invention is to provide a continuously variable traction roller transmission in which a preload as provided by discs springs can be set to a predetermined value with easy adjusting operation.